Section 15Well Disinfection
Chlorine is normally used as the disinfecting agent since it destroys bacteria by neutralising the enzymes that are essential for their survival (Richard, 1987). Chlorine is usually sold in 2 forms: sodium hypochlorite (liquid bleach) and calcium hypochlorite (powder or pellets).
Sodium hypochlorite is the main ingredient in liquid bleaches which initially contain about 5% available chlorine; it gradually loses its strength over time, especially in hot climates. While substantial quantities are required to effectively disinfect large diameter wells, it can be easily mixed with the water and it is relatively safe to handle and use.
Calcium hypochlorite comes in strengths ranging from 30-75 percent available chlorine (70 percent is most common). Like sodium hypochlorite, it slowly loses its strength with exposure to air and should be stored in sealed containers in a cool dark place to retain its strength. Much less quantity of this agent is required to effectively chlorinate wells and special, slow-dissolving pellets can be purchased to provide longer-lasting chlorine residuals. However, calcium hypochlorite becomes unstable and may spontaneously combust if it becomes warm and moist and it can even explode & burn if dropped.
Care must be taken when mixing and adding chlorine to a well since exposure to it can result in severe skin/eye irritations and blisters. It is also poisonous; inhaling concentrations of 30 ppm can lead to harsh coughing and concentrations of 1,000 ppm can be fatal in few breaths!
Chlorine is a very reactive substance. When added to a well, it first combines with inorganic compounds (hydrogen sulphide, ferrous iron, manganese); there is no disinfection at this stage. After these compounds have been reduced, remaining chlorine then reacts with organic matter (algae, phenols & slime growth). While some bad tastes and odours may be eliminated, there is only a slight disinfection action and trihalomethanes (carcinogenic, chlorinated organics) may be formed.
After the demand exerted by inorganic and organic compounds has been met, chlorine will combine with nitrogen compounds (primarily ammonia) to form chloramines. This combined chlorine form results in long lasting disinfection, produces minimal chlorine taste/odour and controls organic growths. However, it is slow acting and long contact times are required.
Finally, once even more chlorine is added to the water, the chloramines are destroyed and excessive chlorine, known as the free residual, forms Hypochlorous Acid (HOCL). HOCL is a potent, fast reacting disinfectant which is desirable when contact times are kept as short as possible and/or if there are high concentrations of iron, manganese, colour or bacteria. The amount of chlorine (dose) required to create sufficient quantities of HOCL depends on:
In established municipal water treatment/distribution systems, water is filtered to remove solids and excessive concentrations of chemicals prior to chlorination. In these systems, where chlorine and water are brought into active contact by pumping into mixers, holding tanks and/or through long distribution lines, initial chlorine dosages are often 1 ppm or less.
However, new wells often have turbid water, elevated concentrations of iron and/or organic slimes and few many existing wells were not thoroughly disinfected following construction and pump installation. Finally, it is difficult to achieve even mixing of chlorine and water in large diameter wells, chlorine tends to settle to the bottom of wells and high chlorine concentrations must reach the outside of the well tiles and the surrounding gravel pack.
As a result, much higher chlorine doses are required for shock chlorination of wells than are used in operating treatment systems. All newly constructed wells should be chlorinated so that a minimal chlorine dosage of 250 ppm is maintained for at least 12 hours (MOEE, 1987). Once wells have been effectively developed and chlorinated, they can be treated by maintaining a chlorine dosage of at least 50 ppm over a contact time of 12 hours. The higher the amount of organics and inorganics in the water, the higher the initial dose must be to ensure that at least 50 ppm chlorine is present in the well 12 hours after it was added.
Wells must be effectively developed prior to disinfection since the presence of organics (including residual drilling fluid) and fine particulate matter can make disinfection incomplete and can result in the formation of compounds that have unacceptable health and/or aesthetic characteristics.
Footnotes & References
1 This assumes a 4 inch dia. ID and a 4.5 inch OD well casing, a 6 inch dia. borehole and a 30% porosity of the gravel pack and/or formation stabilizer in the annular space. The volume of water, therefore, = volume in casing + 0.3 x annulus volume (see Appendix A for volumes). Note that the amount of water within the well casing alone is 8.11 litres per metre of casing below the water table since volume (litres) = depth of water in the well (metres) x (casing diameter in metres/2) x (casing diameter in metres/2) x 1,000 l/m3 x 3.14.
2 If tests show continuing bacteriological contamination, a second chlorination is needed. Following the second treatment additional tests should be conducted. After repeated positive bacteriological tests, a well contractor should be contacted to surge the well and the surrounding formation with a strong chlorine solution. Chlorination will sterilize a well and water system; however, unless the source of the bacterial contamination is found and corrected the problem will continue to reoccur and chlorination will not solve the problem. In some cases, a new well may have to be constructed to correct the problem.